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EU Commentary on EN21 - Total water discharge by quality and destination, including EU Commentary on thermal discharges.

EU21    Total water discharge by quality and destination, including EU Commentary on thermal discharges.

American Electric Power steam electric generating facilities discharged approximately 14 billion cubic meters or 10,281 MGD of process wastewater to surrounding surface waters during 2010 (Table 5), which is less than the estimated 17.4 billion cubic meters or 12,622 MGD discharged in 2009.  This difference is likely due to unit outages or retirements that occurred during 2010.  There is also a discrepancy between the amount of water discharged from AEP facilities and the amount of water that is withdrawn at these same facilities (see EN 8).  Total withdrawals equaled approximately 15 billion cubic meters or 11,020 MGD in 2010, which is just over one-half billion cubic meters or 480 MGD more than what was discharged during the year.  Obviously, there will be differences between years, but another factor to consider is the loss of water due to evaporation in cooling towers and other plant processes.  Total water consumption was estimated to be 0.3 billion cubic meters per year or 198 MGD.

Approximately 98% of the water releases from steam electric facilities are non-contact cooling water, which is used to cool boiler water in condensers (Table 5).  Once-through cooling systems withdraw water from a nearby water body, pass it through a condenser, and discharge it back into the body of water.  Chlorine or other biocides may be added to the water to control biofouling.  In closed cooling systems, water that has passed through a condenser is sent to a cooling tower to lower the temperature.  As water evaporates, the latent heat required to evaporate the water is transferred from the cooling water to the air, cooling the water (USEPA 2006).  Because some of the water evaporates, fresh make-up water is added to the system.  In addition, a small amount of water must be periodically discharged to control the buildup of solids.  This water is referred to as “cooling tower blowdown” (USEPA 2006). 

The next largest wastewater releases are bottom ash and fly ash transport water, however, these effluents combined are only about one percent of all AEP discharges (Table 5).  The burning of coal or oil in steam electric boilers produces a noncombustible residue known as ash.  Heavier particles that collect at the bottom of the boiler are known as bottom ash.  Finer particles that are light enough to be transferred in the flue gas are known as fly ash.  Fly ash and bottom ash can be transported by wet handling systems that produce slurries of ash, referred to as “sluices,” which are typically transferred to wet surface impoundments.  The ash settles in the impoundments prior to recycling or discharge of the water.  Fly ash and bottom ash sluices typically contain heavy metals and inorganic constituents (U.S. EPA 1982).  These values do not exactly match the estimates on Table 2 (134 vs. 158 million gallons per day), since ash pond discharges may also contain storm water and other process waste streams and is not reflected in the “use” estimates.

Other waste streams from AEP facilities include metal cleaning wastes, coal pile runoff, boiler blowdown, FGD chloride purge streams, sump water, turbine seal water, landfill leachate and seepage, and other low volume wastes.  Metal cleaning wastes are those resulting from the cleaning of any metal process equipment.  Chemicals are often used to remove scale and corrosion from boiler tubes.  The major constituents of cleaning wastes are iron, copper, nickel, and zinc.  Alkaline reagents are also used to clean air preheaters and to neutralize acidity.  These alkaline washes can consist of soda ash, caustic soda, phosphates, and detergent.

Coal pile runoff consists of rainwater that has accumulated on and near coal storage piles.  Coal pile runoff is typically acidic and may contain high concentrations of copper, iron, aluminum, nickel, and other constituents present in coal (U.S. EPA 1982).  Boiler blowdown is that water which is periodically discharged from boilers to control the build-up of solids.  There are many sources of impurities in boiler blowdown, including intake water, internal corrosion of the boiler, and chemicals added to the boiler system (U.S. EPA 2006).  Examples of impurities include soluble inorganic salts, calcium, magnesium, iron, copper, chromium, phenol, phosphate, and other chemical species.   Other low volume wastes include laboratory and sampling streams, floor drains, cooling tower basin cleaning wastes, and recirculating service water systems (U.S. EPA 2006). 

All AEP facilities that discharge such effluents have National Pollutant Discharge Elimination System (NPDES) permits that have been issued by the appropriate state agencies.  These permits govern the discharge of the treated wastewaters and ensure compliance with all applicable water quality standards.  The Clean Water Act requires facilities that discharge process wastewaters into receiving waters to control these discharges according to effluent guidelines and water quality-based effluent limits specified in NPDES permits.  The Steam Electric Guidelines specify limits for pH, PCBs, TSS, oil and grease, free available chlorine, total residual chlorine, chromium, copper, iron, and zinc.  These limits are based on the available and economically achievable technologies that can be implemented at steam electric facilities.  Monitoring is conducted at each AEP facility to ensure that the discharges comply with these limits.

Table 5.  Total water discharges from AEP steam electric generating facilities (based on 2010 water withdrawal data).

Facility/Effluent Type Billions of cubic meters/yr Billions of gallons/day

Steam Electric



Non-contact cooling water



Bottom ash transport water



Fly ash transport water



Process water



The majority of water used at AEP generating facilities is used for cooling purposes, either in once-through or recirculating closed systems.  Cooling towers are most frequently used to cool the water in closed systems, however, in both once-through and closed systems, various methods are used to remove biocides and residual oxidants.  Typically, biocides are used in low-level applications to treat the biofouling that occurs in the cooling systems.  Natural decay may be utilized to remove biocides or dehalogenation systems may used to comply with NPDES permit limits.  In these systems, a reducing agent is added to consume the residual oxidizing biocide.  Sulfur dioxide is the most commonly used dehalogenation chemical.  Bentonite clay can be added to absorb excess non-oxidizing biocides, which are not removed by sulfur dioxide. 

Bottom ash and fly ash ponds are used to treat ash sluice water and are primarily settling basins that allow ash constituents and suspended solids to settle out before the transport water reaches the discharge point or is recycled.  Some iron co-precipitation also occurs in these ponds, aiding with the removal of pollutants such as arsenic.  The control of pond pH also helps to precipitate out metals, such as copper.  In some cases, aeration-mixing or treatment chemicals are used to maximize pond effectiveness.

The operation of an FGD system typically results in the generation of a chloride purge stream, which must be treated to manage pH and solids levels.  The treatment process is based on three broad principals:

  • removal of the bulk of the suspended solids in a primary clarification step,
  • conversion of constituents into solid precipitates, and
  • removal of solids remaining after primary clarification, including precipitated solids.

Once treated, this effluent is generally directed to a bottom ash pond for further settling before final discharge to a receiving/source water body.

Source Information - USEPA reports:  USEPA. 1982. Development Document for Effluent Limitations Guidelines and Standards and Pretreatment Standards for the Steam Electric Point Source Category. EPA-440-1-82-029. Washington, DC (November). DCN 03589; USEPA. 2006. Interim Detailed Study Report for the Steam Electric Power Generating Point Source Category. EPA-821-R-06-015. Washington, D.C. (November).   AEP water balance diagrams were used to determine the percentage of water discharged from various waste streams.  These percentages are then applied to water withdrawal information from EN8 to estimate actual amount of water discharged.